Project Summary The goal of this proposal will be to develop a diverse set of new chemical tools centered on tetrazine ligation? the fastest known bioorthogonal reaction. This proposal describes new tools for the safe synthesis and direct coupling of `minimalist' tetrazines to any molecule of interest, including API's and fluorescent reporters. We also propose to develop specialized tetrazines that can serve a dual role in enabling protein purification followed by subsequent site-selective bioorthogonal chemistry. Finally, we propose to develop efficient catalytic methods for `turning on' rapid bioorthogonal chemistry in cellular context. We will develop tool molecules with high stability in the cellular environment in their `off' state, and the fastest bioorthogonal reactions to date in their `on' state. An application of the catalytically inducible tetrazine ligation is new method of drug release where a photocatalyst-antibody conjugate is pretargeted to a disease site, and red light is used to drive the local release of a cytotoxic reagent from a photocaged prodrug.